Many devices are known for display of data, text, and other information. One of the most widely used display devices is a raster scanned cathode ray tube (CRT), wherein an electron beam is focussed and directed onto a phosphor bearing screen by electric and magnetic fields. However, due to aging, temperature variations, differences in terrestrial magnetic field, parameter differences among similar devices, and other causative factors, the raster scan generated by similar CRT displays at different geographical locations may vary, as may the displays generated by a single device change with time.
Such variations in the raster scan of a display result in misalignment of the displayed image, and thus provide degradation of the display. The resulting image misalignment is particularly harmful when the CRT is used in conjunction with a touch control screen (TCS), wherein a touch sensitive panel is overlaid on a CRT and a user inputs signals to a computer controlled system by touching the panel at input locations, or touch keys defined by the image generated by the CRT display. In such an arrangement, misalignment of the image may result in definition of a particular input area at an erroneous location of the touch sensitive panel.
Accordingly, it is necessary to provide correction for alignment errors in a display system, and more particularly to provide misalignment correction for touch sensitive systems.
Realignment has been provided in the prior art by analog methods. One such method utilizes magnets, provided over the back of a CRT, for example, to influence the scanning beam. However, magnetic correction of misalignment suffers from the above described deficiency. That is, magnetic alignment of the display screen is inherently sensitive to the magnetic field of the earth.
Thus, since the magnetic field of the earth varies with location, a correction performed for a specific misalignment at one location may be inappropriate for the same misalignment occurring at another location. Other analog correction methods, wherein synch pulses are delayed, are inherently unstable because of parameter drift with time, temperature, tolerance variation and the like. For these reasons, a uniform realignment procedure cannot be developed for particular misalignment problems.
It is thus seen that prior art realignment techniques require a significant amount of manual skill and dexterity, resulting in increased training expenses for maintenance personnel. Moreover, once aligned, the position of a scanning beam is fixed, and cannot be readily realigned by non skilled personnel. There is accordingly a need in the prior art for realignment techniques which result in stable alignment independent of time, temperature, tolerance variation, or magnetic field, and which are thus also independent of geographical location.
There is moreover a need for realignment apparatus and techniques which may be uniformly applied to display devices, and which may be readily learned by maintenance personnel with a minimum of training.